The orange particles that you found were indeed granules of N350 after being exposed to heat and pressure. Basically, the primer ignited those granules, and the outside layer with the graphite coating burned away, but the powder stopped burning when the bullet left the muzzle and the pressure dropped. If you had put a white bed sheet on the ground out in front of your muzzle, you would have found many more orange granules out there on the sheet.

What Unclenick is saying is that you can expect that to happen when using a relatively slow-burning powder in a relatively short barrel. Sometimes it is "worth it" to get the maximum velocity out of your gun. But, it tends to create a LOT of variation in the actual muzzle velocity of your bullets from round to round. And, in an auto-loader, it can make a mess of the inside of your action and lead to reliability problems until you strip it and clean it out.

So, normally folks choose a powder that will burn completely or nearly completely in the length of barrel that they have. Sometimes that is hard to do with a short-barrelled gun that uses a cartridge that has a large case and a low pressure rating (such as the .38 Special) without loosing a significant amount of muzzle velocity. (Not exactly sure what cartridge you are shooting.)

Anyway, you do not want to reduce your N350 load below the "start" value in the data you are using. That will not get more of the powder to burn; it will make even less of whatever you use succeed in burning, because the pressure will be even lower, and powder burns slower at lower pressure.

If you can increase the charge weight and still stay below "max" in your data, then try that and see how much better it gets the N350 to burn. Of course, don't exceed max. And, don't keep going towards max if you start getting pressure signs. The pressure data accounts for the way the powder burns in your cartridge, so it is not safe to assume that you are getting lower peak pressure than the data indicates just because you are seeing unburned powder. The peak pressures are probably the same no matter how short your barrel is, because the peak is normally reached before the bullet has moved more than an inch or so.